This invention relates to synchronizing signal recovery systems and particularly to a symbol sync recovery system for a VSB (vestigial sideband) digital signal.
The recently adopted advanced digital television signal standards are directed to a multilevel signal having two frames of 313 segments each. There are 832 multilevel symbols in each segment with the first four symbols of each segment defining a two level segment sync pattern and the first segment of each frame including a two level frame sync signal. A small pilot signal is added to aid in carrier recovery in the receiver. The receiver preferably includes a frequency and phase locked loop (FPLL) for recovering the transmitted signal. The data in the transmitted signal is subjected to precoding, interleaving and forward error correction to provide noise protection and to assist in rejection of cochannel NTSC signals.
An important requirement of the receiver is the ability to rapidly and accurately achieve symbol acquisition under field operational-conditions. As with all digital data, the signal-to-noise performance is of great importance. The nominal symbol frequency is 10.76 megasymbols per second and the receiver should ideally acquire symbol frequency lock over a range exceeding +/-70 ppm (parts per million) in under 200 milliseconds. The inventive circuit has this ability.